The yeast, or bacterial, cytosine deaminase converts the innocuous antibiotic pro-drug 5-FC into the cytotoxic chemotherapeutic agent 5-fluorouracil (5-FU). Humans (and mammals in general) are not known to possess a naturally occurring gene encoding an enzyme with significant cytosine deaminase activity. Yeast and bacterial cytosine deaminase have gained recognition in the treatment of cancers using gene delivery and viral vectors for the delivery of the enzyme followed by treatment with 5-FC, which is then converted by the enzyme to a cytotoxic drug (Miller et al., Can Res 62:773-780 2002; Kievit et al., Can Res 59:1417-1421 1999).